A Solar cell has advantages that the amount of carbon dioxide emitted per power generation is small and that no fuels for power generation is required. Thus, studies on various types of solar cells have been actively developed. Currently, among the solar cells in practical use, a mono-junction solar cell having a set of p-n junction and using single-crystal silicon or polycrystal silicon has become mainstream. However, the theoretical limit of the photoelectric conversion efficiency of the mono-junction solar cell (hereinafter, referred to as a “theoretical efficiency limit”.) remains at about 30%; therefore, new methods for further improving the theoretical efficiency limit have been studied.
One of the new methods which have been studied so far is a solar cell employing a quantum structure of a semiconductor. Known examples of the quantum structure used for this type of solar cell include a quantum dot, quantum well, and quantum wire. By employing the quantum structure, it is possible to absorb a bandwidth of the solar spectrum which cannot be absorbed by a conventional solar cell. Therefore, it is assumed that with a solar cell employing the quantum structure, the theoretical efficiency limit can be improved up to 60% or more.
As a technique related to such a solar cell (including an optical semiconductor device), for example, Patent Document 1 discloses an optical semiconductor device wherein a semiconductor light-emitting/receiving element comprises as its light-emitting/receiving layer, a plurality of layers comprising a quantum dot made of a semiconductor and having a cross-sectional size of approximately the de Broglie wavelength of the electrons; and comprising a semiconductor surrounding the quantum dot and having a potential energy higher than that of the quantum dot to function as an energy barrier. Further, Patent document 2 discloses a solar cell formed by a p-i-n structure, and comprising a quantum dot which has a three-dimensional confinement effect in an i-layer as a photodetecting layer, wherein the energy band structure of the quantum dot and the barrier layer surrounding the quantum dot is a Type-II; a layer for preventing injection of a carrier which serves as a barrier for preventing injection of a hole into the quantum dot, is formed on an n-layer side of the quantum dot; and a layer for preventing injection of a carrier which serves as a barrier for preventing injection of an electron into the quantum dot, is formed on a p-layer side of the quantum dot.